Commonly in sewing machines, after the execution of a stitch the article to be sewn is pushed forward to the subsequent stitching position via a transportation device. Frequently, transporters arranged in the area of the stitching plate are used for this purpose, which execute a flat-oval motion and hereby protrude through slits in the stitching plate in the driving phase and displace the article to be sewn in this manner. Several different methods are known to supervise the article to be sewn during such transportation processes. For example, sensors may be provided, which detect the edges of the article to be sewn and interrupt the sewing process in due time preventing any stitching from occurring outside the article to be sewn.
From DE 19850742, a device is known having a camera, which is used to determine the position of two adjacent stitching sites of the sewing needle on the article to be sewn. A reference device determines any deviations of the actual values from the predetermined target values for the position of these stitching sites and influences the material feed such that subsequent stitching sites deviate as little as possible from each desired target position.
When quilting, the article to be sewn, resting on the material support, is generally displaced manually into different directions, i.e. without the help of a material transportation device. Conventionally, the needle rod motion for executing the stitch is controlled by the operating person manually or via a treadle starter. The operating person himself/herself must adjust the sewing speed such that the respective speed of the material feed is adjusted such that the individual sewing stitches are executed at even distances in reference to one another. However, a method is already known, according to which a sensor arranged below the material support detects relative movements of the sewing machine and/or an object, immobile in reference to the sewing machine, to the material layer directly resting on the material support and/or stretched in a frame, controlling the sewing needle drive such that the distance between the individual stitches remains as constant as possible.
Generally the article to be sewn comprises several layers of material laying on top of one another and/or an application attached to a material layer. When displacing such a material stack comprising two or more material layers the position of the individual layers in reference to one another may change due to slippage. The extent of the slippage depends on the nature of the individual material layers, thus on the materials and their thickness, for example. Also, the number of individual layers is influential.
When determining the feed of the material and/or the article to be sewn from below only the movement of the lowermost material layer is registered. Due to slippage the uppermost layer can be displaced in reference to the layer detected by the sensor, which is the visual side after the sewing and/or quilting process. In the uppermost, visible material layer therefore the individual stitching positions can deviate from the desired target position. The limited spatial conditions in the area of the stitching plate below the material support oppose an optimum embodiment of the sensors for determining the movement of the article to be sewn. Unduly high faults can occur in detection sites at a great distance from the stitching site of the sewing needle. When the detection of the movement of the article to be sewn is to occur as close as possible to the stitching site of the sewing needle, this may prohibit the simultaneous presence of a transporter in this area.
Furthermore, the sharpness of conventional optical detection devices for detecting the movement of article to be sewn is extremely low. Therefore, in order to flawlessly detect the article to be sewn, means have to be provided, by which the article to be sewn can be held inside the limited range of sharpness of the detection device.